Mononitrobenzene is produced industrially using a number of adiabatic nitration technologies. Adiabatic nitration was first introduced by Castner, as described in U.S. Pat. No. 2,256,999. Adiabatic nitration of benzene has replaced the previously-used energy-intensive isothermal nitration process.
In the late 1970's Alexanderson et al. proposed changes to the composition and temperature limits of Castner's adiabatic nitration process, leading to a commercially useful process, described in U.S. Pat. Nos. 4,021,498 and 4,091,042. U.S. Pat. No. 4,021,498 describes a process for making mononitrobenzene using a mixed acid having a nitric acid strength of 5 to 8.5 wt %, a sulfuric acid strength of 60 to 70 wt %, and not less than 25% water, the initial reactant temperature being in the range of 40 to 80° C. U.S. Pat. No. 4,091,042 describes an improved process using a mixed acid having a nitric acid strength of 3 to 7.5 wt %, a sulfuric acid strength of 58.5 to 66.5 wt %, and the balance water. The temperature of the initial mixed acid is in the range of 80 to 120° C. Compared with the prior art isothermal technology these conditions led to a reduction in the production of the by-product dinitrobenzene, stated as less than 500 ppm in the '042 patent. However, the conditions described in the Alexanderson et al. patents still produce a high level of by-product nitrophenols.
Since the 1990's, most new industrial mononitrobenzene adiabatic plants have been built based on process conditions described by Guenkel et al. in U.S. Pat. No. 5,313,009. Guenkel et al. proposed a new set of conditions to nitrate benzene which reduced formation of oxidation by-products (i.e., nitrophenols) below prior art levels. In the Guenkel et al. process the initial mixed acid temperature must be in the range of 97° C. and 120° C. Reaction rates are stated to maintained high by keeping the reactor inlet mixed acid temperature high (i.e., above 97° C.) and by maintaining the sulfuric acid strength relatively high. The by-product nitrophenol formation for the conditions described by Guenkel et al. is in the order of 1700 ppm. Experiments have shown that the by-product dinitrobenzene formation under the conditions described by Guenkel et al. is in the range of 250 to 300 ppm.
Nitrophenols and dinitrobenzene are the main impurities formed in the industrial production of mononitrobenzene. Reducing the levels of these impurities is a very important goal in the art. Most industrially-produced mononitrobenzene is used in the production of aniline, and these impurities are believed to be the main compounds negatively affecting catalyst life in the downstream aniline process. The operating cost and capital investment required to remove and treat these by-products is significant.